Rehma Sabir was allegedly attacked on her first birthday in the family’s American home by Aisling McCarthy Brady, who was working in the US illegally and had a history of alleged violence.

Rehma suffered brain damage in the assault, prosecutors say, and died two days later in hospital in Boston, Massachusetts.

The baby’s London-born father, Sameer Sabir, and mother, Nada Siddiqui, were last night being comforted by Mr Sabir’s parents, who travelled to the US who had travelled from Thornton Heath, south London, to the US to celebrate the baby’s birthday.

Mrs McCarthy Brady, 34, has been charged with assault and battery on a child causing substantial bodily injury. Prosecutors said this may turn into a murder charge after full post mortem examination results are considered.

Gerry Leone, the district attorney for Middlesex County, said the baby’s injuries indicated that she suffered “abusive head trauma”. He described her death as “an extremely troubling case”.

“We allege the defendant violently assaulted a one-year-old child, causing a devastating head injury and broken bones,” said Mr Leone.

Prosecutors said doctors also found evidence that Rehma was recovering from existing fractures to her left arm and leg when she died.

Mrs McCarthy Brady, of nearby Quincy, denied all charges in a hearing at Cambridge district court, where she hid from public view.

Her lawyer, Melinda Thompson, said that she had “loved” Rehma, and claimed that the child had returned malnourished from trips with her parents to London and Pakistan.

The nanny, who was previously arrested for alleged assault and was the subject of two restraining orders, was detained pending a court hearing next month.

Bail was set at a $500,000 (£315,000) cash bond, with demands that she surrender her passport and be placed under house arrest with a GPS tracking device.

Police said that she had been alone with Rehma from 9.30am on Jan 14, after Ms Siddiqui left the family’s home in the affluent Boston suburb of Cambridge.

Mr Sabir, 34, moved to the city from Britain for his postgraduate studies at Harvard University and Massachusetts Institute of Technology in 2008.

His wife, who was born in Pakistan, works as a chartered financial analyst for an asset management firm in Boston. The 29-year-old graduated with a degree in science and engineering from New Jersey’s Princeton University.

Officers arrived at the couple’s home shortly before 5pm on Jan 14, after receiving a 911 call from Mrs McCarthy Brady, who is originally from Co Cavan. Rehma, who was found to be unconscious, was taken to hospital, where she was treated for bleeding to her brain and eyes. She was declared dead two days later.

Police discovered traces of blood on the pillow and blanket in her crib, and on discarded baby wipes, according to court documents. They also reported finding that a wall was damaged by “forceful contact” with the baby’s changing table.

There was no answer last night at Mr Sabir and Ms Siddiqui’s rented condominium, which had received deliveries of flowers and condolence messages.

Mr Sabir is the founder and president of MoMelan Technologies, which develops devices for carrying out skin grafts. He graduated from University College London with a degree in immunology before spending three years as an investment banker at JP Morgan.

He also worked as a management consultant before studying for a master’s degree in business in Massachusetts.

A friend of Sameer Sabir, who declined to be named, last night told The Daily Telegraph he had been looking forward to parenthood.

"He had his first baby so he was in pretty good shape as business also seemed to be going well also," he said.

"He is very smart guy. What he has done in Boston is hugely exciting. It is just terribly tragic."

The couple married in a lavish ceremony at the Thistle Hotel, by Tower Bridge, central London a few years ago attendede by "several hundred" people, where the "food was superb and the Bollywood dancing even more amazing".

Neighbours in South London last night expressed their shock over the death.

Mr Sabir’s parents, Naveed, 63, who worked from home, and Seema, a 57 year-old civil servant, told friends of their joy at flying to America for their only grandchild's birthday.

When the baby fell ill and "could not wake up", Mr Sabir's sister Erum, 32 and brother Osman, 28, flew out of London to be by their sibling's side.

When the young couple visited London in November, the baby was "normal" and "healthy".

She said when the couple visited in November the baby was starting to talk, crawl and was just about to begin walking.

Mr Sabir then made another trip to London just before Christmas for a family friend's wedding.

Close friend Sameena Chaudhary, a 59 year-old nursery school teacher, who had known Mrs Sabir for almost 25 years, said the family were devastated.

She said: "It is very sad. They are all in shock, deeply.

"We were all praying for Rehma. We were all in a bad situation.

"They were proud grandparents. They are a very close family.

She added: "We can't speak to them because they are so upset. We can't think how it happened.

"It is shocking news. For something like that to happen is just awful."

She said the grandparents were due back in London [Wednesday] but have delayed their return in order to comfort the couple.

"They [the grandparents] were already there in America. They phoned Osman and Erum to come to Boston because something has happened and the baby was not well. The were all in the hospital."

Another neighbour, who declined to be named, added: “They were very happy, they just loved being grandparents,” said one, who declined to be named.

“Their son is very successful in America and they spoke so highly of him. They were so proud. They absolutely loved having a grand-daughter."

US immigration officials said Mrs McCarthy Brady had outstayed a 90-day visa waiver in 2002. She is believed to have married Don McCarthy, a 38-year-old, Irish-born painter, last year.

On Wednesday night Mrs McCarthy Brady’s upstairs neighbour said that he knew “what she’s capable of” based on the “anger that she would display” around the area.